I see no reason not to allow players to play good-aligned assassins, who kill evil NPCs for example.
But then they are doing it for a good cause and not for purely personal benefits. Which means they are not assassins.
For better or worse, that seems to be part of the history of D&D. Paladin = front loaded 'good' motivation (honor, etc.). Ranger's bonus attack to favored enemy = front loaded motivation against a specific species. And of course, assassin comes with front loaded baggage and cognitive dissonance regarding the purpose and motivation of the character. For me, sniper is simply not the same as assassin in terms of implying a motivation.Does the game really have to front-load what a Classes' motivation is, rather than leaving it to the person that is actually supposed to be playing it.
I think it would help a lot if the game didn't use terminology and flavor or mechanics that can induce the impression of front-loading motivation (or found a compelling way to reboot or reinvent the class concept). Considering that 5E is trying to unify all editions, and considering that every edition comes with different preconceptions, I'm sure Mike and Mearls are probably asking themselves these kinds of questions.
OK, I saw it, it was a parody of a sniper, and a video game character at that. Why did we "need" see to that -- is this supposed to reinforce that a D&D character must be like a videogame character?Everyone needs to go watch the Team Fortress 2 video "Meet the Sniper."

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.